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Citation: Spina R. Pockmarks as indicators to decipher some natural phenomena in the field of geology and beyond: state of knowledge and its implications. J Environ
Geol. 2018;2(S1):1-2.

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The subject matter of the first special issue of the “Journal of Environmental
Geology” was focused on pockmarks, ie craters in the seabed caused
by fluids [gas and liquids] erupting and streaming through the sediments
generating the moon-like terrain on parts of the ocean floor.

Thanks to the development of technological means [multibeam echosounder
and satellite images] pockmarks have been found in the seabed of worldwide
and have assumed considerable importance in different geological sectors
both for the possible role of precursors of natural disasters [1-3] and for
the important information provided on the circuits of pressurized fluids
hydrocarbons [4,5] and hydrate gases [6,7] and on the ascent of salt diapirs
[8]. Furthermore, their distribution is often not random but is the reflection
of endogenous movements: in some cases, are developed along preferential
directions due to the existence of deep tectonics discontinuity [9,10] or
radially distributed with respect to a center of symmetry (Figure 1) [11].

Figure 1: View of a symmetrical pockmark star with a flat central area [11]

In recent years, several studies consider pockmarks, and associated structures
[seepages, mud volcanoes], as sources of useful information to decipher
some current and past geological phenomena [1-3,12]. Pockmarks dimension
ranging in size from the ‘unit pockmark’ [1–10 m wide, <0.6 m deep] to the
normal pockmark [10–700 m wide, up to 45 m deep] are known to occur in
most seas, oceans, lakes and in many diverse geological settings [1].

The pockmarks play a very important role also in the field of scientific studies
aimed at economic purposes: the presence of methane emitted spontaneously
[fluid could seeps] can indicate the existence of a heavy hydrocarbon
reservoir near the gaseous reservoir and, in this way, the pockmarks can be
useful indicators of the existence of petroleum. For this reason the majority
of pockmark research has been driven by the oil and gas industry, in terms
of their usefulness as an exploration tool [4] or as indicators of hydrocarbon
sources for prospecting [5].

Pockmarks can be associated with the rise of hydrate gases that contain highly compressed natural gas that may constitute a significant source of
energy: some nations [USA, India, Japan, South Korea, and China] they
funded research programs in the field of hydrate gases to start commercial
production of gas from hydrates. In the Barents Sea significant emissions of
methane have been documented with pockmarks of diameter between 300
and 1000 m and about 30 m deep, alternating with mounds present in the
sea bottom about 1 km wide and 20 m high. It is assumed that during the
glaciation natural gases migrated from the underlying hydrocarbon reservoirs
and were trapped inside the ice molecules, in metastable conditions, forming
the so-called hydrate gases. Upon ice sheet retreat, methane from gas hydrate
reservoirs concentrated in massive mounds before being abruptly released to
form craters [6]. Pockmarks can therefore be considered useful indicators in
the identification and exploitation of hydrate gas fields [7].

In some cases, the sediments present inside the pockmarks can be of great
interest to reconstruct the paleoceanographic and paleoclimatic conditions
of the past: in areas characterized by extensive methane seepage from the
ocean floor, it is possible to study the interaction between the releases of
methane correlated with climatic changes throughout time. Potential seepage
of gas is important to study in order to investigate the possible effects it might
cause on a continuously changing climate [13].

One of the most important topics on which the special issue has been
focused is the role of valid precursors of high magnitude seismic events:
there are several observations suggesting that pockmarks go into an active
state before and during earthquakes, including a generally higher rate of
gas venting from the seafloor during earthquakes. On the occasion of the
earthquake in Patras [Greece] on July 14, 1993 [Magnitude 5.4] the water
temperature rose from 16.8°C to 23°C for 5h 25 min at 01.30. Prior the
earthquake the hydrographic recorder had sensed three bursts of abnormally
warm water in the Gulf. On inspection with side-scan sonar and sub-bottom profiler, pockmarks up to 15 m deep on the bottom of the gulf were found
to be venting gas [1,3].

There are other similar events with sea water heating and an increase in the
emission of gas from the pockmarks before the earthquake, such as in the
case of the 1980 California earthquake [1] and Aigion earthquake in 1995
[1,2].

Pockmarks also affect the ecology of the seabed [14]: some scientists,
comparing macrofaunal assemblages inside and outside of pockmarks,
have found important but subtle differences to those on non-pockmarked
substrata [15].

The pockmarks-based articles only in recent years, thanks to the improvement
of the bathymetric surveys, have had a significant increase. The scanning of
ever larger portions of the seabed has allowed us to discover new structures
and assign new meanings to different phenomena closely related to the earth
sciences [petroleum geology, geophysics,...] and similar [for example biology].

The recent discover of the giant pockmarks (Figure 2) [16-18] and the
particolary structures seastar-shaped in the seabed surrounding Hawaii
islands [11] have given and will give more importance to the study of
structures pockmarks-based.

Figure 2: Three dimensional image of the giant pockmarks discovered in the seabed on the southern flank of the Chatham Rise seafloor (New Zealand). Legend, on the left, indicates the bathimetry of the sea (Courtesy Research Expedition SO226-2, 2013)

Arvo J. Relationship between fluid leakage and faulting along the western and northern margin of the Hammerfest Basin. The Arctic University of Norway, Department of Geology. Master thesis in GEO-3900. 2014.